The present invention relates to an electron gun for producing and directing a plurality of electron beams, and more specifically to the means for focusing the beams on a target.
In a conventional color television picture tube having a multiple beam electron gun, each electron beam is passed through a separate electron lens which focuses each beam to a point on the target screen. The lens is essentially an electrostatic field which deflects the individual rays of the electron beam toward a common point as they pass through the lens. This field is normally established between two spaced electrodes positioned transverse to the beam paths. The electrodes have a series of apertures through which the electron beams pass. The characteristics of the lenses may be altered by changing the electrostatic field, which is usually accomplished by varying the voltage between the electrodes, the size of the apertures, the separation distance of the electrodes, or a combination of the above.
In some cases in order to reduce spherical aberration, it is desirable to have a long focal length electron lens. Since the voltage of the focus electrode must be restricted to values which do not cause arcing at the picture tube base, the focal length can most easily be lengthened by increasing the aperture size and/or the electrode spacing. However, if the spacing between the electrodes becomes too large (in excess of about 1.5 mm), the electrostatic focusing field becomes susceptible to interference from other electrostatic fields within the electron gun. Conversely, each focusing field created by the large spacing also interferes with the adjacent electrostatic focusing fields. Ideally therefore, the electron lens should have a relatively large diameter aperture, about 8 mm and a small electrode spacing about 1.5 mm.
The physical design of the electron gun also places several restraints on this configuration. In an in-line electron gun, as shown in U.S. Pat. No. 3,772,554 issued on Nov. 12, 1973 to Richard Hughes, there is a very close spatial relationship between the electron beams. Due to this close relationship, the lens characteristics may be adjusted only by varying the spacing of the electrodes since the aperture size is already maxmized. Therefore, in order to duplicate the characteristics of the ideal large aperture lens in an in-line electron gun, the electrode spacing must be several times the maximum tolerable limit of 1.5 mm.